The invention relates to semiconductor devices and, in particular, to high power diodes.
One structure for a semiconductor diode, such as a Schottky rectifier, includes a semiconductor layer onto which two metal contacts, including an ohmic contact and a Schottky contact, are formed. In high-power applications, these diodes must be able to withstand relatively large reverse-bias voltages without breaking down, i.e., without conducting current in the reverse direction. One factor that influences the reverse breakdown voltage is the width of the depletion region in the semiconductor layer. In general, a wider depletion region, and thus a wider diode, has a higher reverse breakdown voltage.
The doping level of the diode""s semiconductor layer also influences its reverse breakdown voltage. In general, a lower doping level leads to a wider depletion region, which, in theory, leads to better performance in high power applications. However, reducing the doping level also increases the diode""s on-state resistance, i.e., the resistance of the diode when conducting in the forward-bias mode. Greater on-state resistance leads to increased voltage drop across the diode, which limits the diode""s utility and efficiency in many applications. As a result, reductions in doping level and corresponding gains in reverse breakdown voltage are limited by the acceptable on-state resistance of the diode.
The inventors have developed a high-power device that withstands relatively large reverse-bias voltages with relatively low on-state resistance. This device exhibits an on-state resistance that is often many times smaller than the on-state resistances of similarly sized devices with similar reverse breakdown characteristics. For example, one particular device, described below, has an on-state resistance that is approximately 30 times smaller than the on-state resistance of a conventional device of similar size having a similar reverse breakdown voltage.
In one aspect, the invention features a semiconductor diode that includes at least two stages, each having a semiconductor layer and two conductive contacts. The stages are arranged such that a contact on one of the stages and a contact on another of the stages are joined together.
In some embodiments, the two contacts on each stage are formed from different materials. For example, a metallic material with a high metal work function, such as gold, palladium, platinum, or rhenium, often is used to form an ohmic contact on each stage, while a metallic material with a low metal work function, such as titanium or titanium aluminum, is used to form a Schottky contact. The semiconductor layers are formed from any of a wide variety of materials, including nitride-based materials, such as GaN and AlGaN, and carbide-based materials, such as SiC. In some embodiments, the materials used to form the semiconductor layers vary from stage to stage. In other embodiments, the semiconductor layer in one or more of the stages has a varying doping concentration.
In another aspect, the invention features a Schottky rectifier having multiple stages with substantially identical or very similar structures. Each stage includes a nitride-based semiconductor layer, a Schottky contact formed on one surface of the semiconductor layer, and an ohmic contact formed on an opposite surface of the semiconductor layer. The Schottky layer is formed from a metallic material with a high metal work function, and the ohmic contact is formed from a metallic material with a low metal work function. At least one of the stages is a middle stage located between two adjacent stages, such that the Schottky contact of the middle stage and the ohmic contact of one of the adjacent stages are joined together, and such that the ohmic contact of the middle stage and the Schottky contact of another one of the adjacent stages are joined together.
Other aspects of the invention include techniques for use in fabricating high power Schottky rectifiers. One of these techniques includes forming a conductive contact layer on a semiconductor substrate, forming a semiconductor layer on a transfer substrate, and then forming a conductive contact layer on the semiconductor layer. The two conductive contact layers are then placed in contact with each other and bonded together. Upon removing the transfer substrate, two additional conductive contact layers are formed, one on an exposed surface of the semiconductor layer, and one on an exposed surface of the semiconductor substrate.
In some embodiments, a metallic material with a high metal work function is deposited on the semiconductor substrate to form a Schottky contact, and a metallic material with a low metal work function is deposited on the semiconductor layer to form an ohmic contact. Techniques for bonding the contact layers together include running an electrical current through the contact layers and applying heat to the contact layers.
Another technique for use in fabricating a high power Schottky rectifier includes forming a first conductive contact layer of a first type (e.g., a Schottky contact) on a semiconductor substrate and forming a second conductive contact layer of a second type (e.g., an ohmic contact) on the first conductive contact layer. Another semiconductor layer then is formed on the second conductive contact layer. A third conductive contact layer of the first type (e.g., Schottky) is formed on an exposed surface of the semiconductor layer, and a fourth conductive contact layer of the second type (e.g., ohmic) is formed on an exposed surface of the semiconductor substrate. The resulting structure is a rectifier having multiple stages.
Other embodiments and advantages will become apparent from the following description and from the claims.